If you haven't already you could try and install the firmware via ethernet maybe?
Reboot the router first and then try the upgrade, also try using Edge / Chrome browser without any extensions.
The 4.8.3 will just mainly have fixes and is the recommended stable build.
You can always check the release notes to find out what's changed.
Here are the release notes for 4.8.3
The 4.8.99 build is a beta build with some exciting new features like DPI (deep packet inspection) which will eventually be coming to the stable build but for now it's a time based trial build whereby the stats / DPI features will stop working. Give it a try, you can always revert to the stable release.
Check out this post regarding the .99 beta
Release notes for 4.8.99